The present invention relates to a system for osteosynthesis at the vertebral column, in particular for stabilization of spinal segments, to a connecting element for such a system, and to a tool for placement and removal of such a system.
In EP 0 441 729 A1 is disclosed such a system for osteosynthesis at the vertebral column, in particular for stabilization of spinal segments. This system comprises at least one rodlike connecting element and at least two retaining means, each of which can be anchored to a spinal segment, with a tuning-fork-like head, the two limbs of which delimit an approximately U-shaped receptacle for the connecting element. In addition this system comprises a threaded stopper, which can be screwed into the receptacle in order to fix the connecting element between the two limbs of the tuning-fork-like screw head. The floor of the receptacle is concave, and disposed between the floor of the receptacle and the connecting element is a round split ring the bearing surface of which, facing toward the floor of the receptacle, has a complementary convex shape. This system is intended to counteract the disadvantage of conventional systems, which ultimately derives from their special construction. That is, it is necessary for each of the retaining means to be very precisely placed at the corresponding spinal segment so that the connecting element can ultimately be mounted and permanently fixed to the at least two retaining means. In practice, however, the manipulation of this system according to EP 0 441 729 A1 has proved to require a relatively great effort during the surgical operation.
Other such systems for stiffening a part of the vertebral column comprising at least two vertebrae are known in the state of the art. These systems each comprise at least two screw-shaped or hook-shaped retaining means, each of which can be attached to one of the vertebrae of the affected part of the vertebral column. The retaining means are provided to receive and fix in position the connecting element that connects the retaining devices with one another. This arrangement is intended to achieve stiffening or so-called spondylodesis of individual parts of the vertebral column in cases of spinal curvature (scoliosis, kyphosis), injury (trauma), neoplasms (tumor) and in particular abrasion or degenerative spinal disease, according to the principle of osteosynthesis under conditions of the greatest possible rigidity, i.e. extreme mechanical stability.
The retaining means in these systems as a rule are screws, or so-called pedicle screws, or also special hooks, which in all cases are connected to one another by a mechanically stable connecting element in the form of a bending-resistant rod, a threaded rod or the like. To achieve a sufficiently great clamping action, the connecting element, which usually has an angular or shaped form, is received in conical cut-outs in the heads of the pedicle screws or hooks. However, these systems require the connecting element in its bearing to contact the pedicle screws or the hooks by way of a flat surface, so that an optimal clamping action is achieved on the basis of at least three-point contact. In practice it has turned out that in these systems the bearing surface is so small that the load is applied at only one point. Such punctate loading, during the everyday micromovements of the vertebral column, causes locally restricted, particularly high strain within the implant as a whole, which in turn causes loosening of the whole implant, wear with possible friction corrosion, deformation or even breakage. Another disadvantage of these systems is that they are relatively difficult to manipulate during the surgical operation, in order to achieve a precise fit and hence a strong clamping action. Often, therefore, it is hard to carry out a correction during the operation. The consequence, again, is a reduced constancy of the operation quality. Finally, the comparatively elaborate construction of these systems is disadvantageous, because on the one hand it limits their applicability for various indications and on the other hand it greatly increases the amount of space they take up, so that there is room for only a slight covering of muscle and skin.